As shown in a schematic view in FIG. 11, a known propeller windmill used in wind power generators includes, for example, three blades composed of a first blade 1, a second blade 2, and a third blade 3, a rotor 5 serving as a link mechanism for linking the three blades, a tower 4, and so on.
In general, each of the blades of such a propeller windmill is controlled depending on the wind conditions so as to obtain a predetermined rotational speed and output of a power generator.
FIG. 12 shows an example of the structure of a known pitch-angle control device. As shown in the figure, the known pitch-angle control device includes a common-pitch-angle command-value generating unit 15 for generating a common-pitch-angle command value on the basis of the difference between a preset value of a rotational speed or output of a power generator and a controlled value at that time. Actuators control each blade so as to have identical pitch-angles on the basis of the common-pitch-angle command value generated by the common-pitch-angle command-value generating unit 15, thus controlling the pitch-angle of the blades.
The inflow wind speed to a windmill is affected by the ground, as shown in FIG. 13A (the wind speed characteristics affected by the ground are hereinafter referred to as “wind shear characteristics”), or by the tower supporting the windmill, as shown in FIG. 13B (the wind speed characteristics affected by the tower are hereinafter referred to as “tower characteristics”). Spatial disorder and temporal disorder of the wind speed are added to the effects described above, resulting in an uneven wind speed distribution in the blade rotation area, as shown in FIG. 13C. Under such uneven wind speed conditions, since the instantaneous values of the aerodynamic output from each of the blades are different from each other, the values of the thrust, the moment, and the like of the blades are also different from each other. As a result, a load fluctuation in each blade occurs, thereby shortening the lifetime of the blades.
To overcome this problem, for example, PCT Japanese Translation Patent Publication No. 2001-511497 discloses a technology in which the angle of attack of wind flowing to each blade and the load are measured, and the blades are individually controlled on the basis of these values.
PCT Japanese Translation Patent Publication No. 2001-511497.
PCT Publication No. WO01/86141 pamphlet